The present invention is related to keyboard electronic musical instruments, such as electronic organs, and in particular to a system for producing a monophonic binary signal representative of a selected depressed key on the keyboard wherein this signal is utilized to control the tone generation and keying circuits of the organ, as, for example, in the production of brass tones.
Since the tonal quality of a note produced by a musical instrument is greatly dependent upon the harmonic content of the wave form, in electronic organs it is customary to generate rectangular waves which are then altered or filtered to produce wave forms having the desired harmonic content. The duty cycle of the rectangular waves has a very pronounced effect on harmonic content, with short duty cycles, that is, a narrow pulse width, sounding more brilliant, and a longer duty cycle sounding more mellow. An example of a more brilliant sound is that produced by a trumpet, whereas a saxaphone produces a more mellow tone.
A characteristic of the sound produced by brass instruments and some woodwind instruments is that during the attack portion of the tone, the amplitude increases and the tonal quality becomes more brilliant. In the case of square wave tones, this means that the duty cycle decreases with time during the attack. This sound may be simulated in a electronic organ by causing the tone to gradually increase in amplitude when the key is depressed with a gradual decrease in duty cycle. The duty cycle may either decrease simultaneously with the increase in amplitude, or lag the amplitude increase somewhat so as to simulate the effect which is produced when a mute is used with the trumpet or trombone. During decay, the pulse amplitude will decrease and the duty cycle will increase, so that the tone will decay and become more mellow.
Although a number of techniques for producing brass tones have been developed, they are often quite expensive to implement and are, thus, of limited use in a small organ format. Often, the prior art brass systems are responsive to polyphonic signals, wherein a plurality of the keys are simultaneously depressed. Since brass tones played as chords often sound muddy and unclear, brass is normally played monophonically, with one key being depressed at a time. Many prior art systems for converting the polyphonic output of a keyboard to a monophonic signal have been developed, and, although such systems would be useful for developing monophonic brass, this would restrict the flexibility of the overall organ. This is especially true in the case of synthesizers wherein it is often desirable to differently voice portions of chords played on the solo manual. If the keyboard output were only monophonic, then this technique would not be available.
Another problem is that of interfacing, in a cost effective manner, an inexpensive brass system in a multiplexed organ without substantially redesigning the existing organ circuitry. It is also desirable that any subsystem which is added to the organ be capable of adaptation to other uses simply and effectively.